A variety of methods are used in the semiconductor manufacturing industry to deposit materials onto surfaces. For example, one of the most widely used of such methods is chemical vapor deposition (“CVD”), in which atoms or molecules contained in a vapor deposit on a surface and build up to form a film. Deposition of silicon containing materials using conventional silicon sources and deposition methods on certain surfaces, such as insulators, is believed to proceed in several distinct stages. Nucleation, the first stage, occurs as the first few atoms or molecules deposit onto the surface and form nuclei. Nucleation is greatly affected by the nature and quality of the underlying substrate surface. During the second stage, the isolated nuclei form small islands that grow into larger islands. In the third stage, the growing islands begin coalescing into a continuous film. At this point, the film typically has a thickness of a few hundred angstroms and is known as a “transition” film. It generally has chemical and physical properties that are different from the thicker bulk film that begins to grow after the transition film is formed.
In some applications, it is desirable to achieve uniform or “blanket” deposition over both insulating (for example, silicon oxide) and semiconductive (for example, silicon) surfaces. In other applications, it is desirable to deposit selectively on semiconductor windows exposed within fields of different materials, such as field isolation oxide. For example, heterojunction bipolar transistors are often fabricated using selective deposition techniques that epitaxially deposit single crystal semiconductor films only on active areas. Other transistor designs benefit from elevated source/drain structures, which provide additional silicon to be consumed by the source/drain contact process, thus leaving the performance of the resulting shallow junction device unaffected. Selective epitaxy on source/drain regions advantageously allows the number of subsequent patterning and etching steps to be reduced.
Generally, selective deposition takes advantage of differential nucleation during deposition on different materials. Selective deposition generally involves simultaneous etching and deposition of the material being deposited. The precursor of choice generally has a tendency to nucleate and grow more rapidly on one surface and less rapidly on another surface. For example, silane will eventually deposit silicon on both silicon oxide and silicon, but there is a significantly longer nucleation phase on silicon oxide. Thus, at the beginning of a nucleation stage, discontinuous films on oxide have a high exposed surface area relative to merged, continuous films on silicon. Accordingly, an etchant added to the process will have a greater effect upon the poorly nucleating film over oxide as compared to the rapidly nucleating film over silicon. The relative selectivity of a process is thus tunable by adjusting factors that affect the deposition rate (for example, precursor flow rate, temperature, and pressure) and the rate of etching (for example, etchant flow rate, temperature, and pressure). Changes in variables such as these generally result in differential effects upon etch rate and deposition rate. Typically, a selective deposition process is tuned to produce the highest deposition rate feasible on the window of interest while accomplishing little or no deposition in the field regions. Known selective silicon deposition processes include reactants such as silane and hydrochloric acid with a hydrogen carrier gas.
A variety of approaches have been used to make strained single crystalline silicon containing materials that have applications in the semiconductor industry. One approach involves developing the strain at the substrate level before the device (such as a transistor) is fabricated. For example, a thin single crystalline silicon layer can be provided with tensile strain by epitaxially depositing the silicon layer on a strain-relaxed silicon germanium layer. In this example, the epitaxially deposited silicon is strained because its lattice constant follows the larger lattice constant of the underlying silicon germanium. Tensile strained epitaxially deposited silicon typically exhibits increased electron mobility.
Another approach for fabricating strained silicon crystalline silicon containing materials is by substitutional doping, wherein the dopants replace silicon atoms in the lattice structure. For example, substitution of germanium atoms for some of the silicon atoms in the lattice structure of single crystalline silicon produces a compressive strain in the resulting substitutionally doped single crystalline silicon germanium material because the germanium atoms are larger than the replaced silicon atoms. Alternatively, tensile strain is provided in single crystalline silicon by substitutional doping with carbon, because carbon atoms are smaller than the silicon atoms that they replace.